1. Field of the Invention
The present invention relates generally to the structure of radially expansible lumenal prostheses, including stents and grafts. More particularly, the present invention relates to the structure of prostheses having controlled expansion characteristics and controlled radiopacity.
Lumenal prostheses are provided for a variety of medical purposes. For example, lumenal stents can be placed in various body lumens, such as blood vessels, the ureter, the urethra, the biliary tract, and the gastrointestinal tract, for maintaining patency. Such stents are particularly useful for placement in pre-dilated atherosclerotic sites in blood vessels. Lumenal grafts can be placed in blood vessels to provide support in diseased regions, such as abdominal and other aneurysms.
Lumenal stents and other prostheses are available in a wide variety of configurations, including helical coils, rolled tubular sleeves, serpentine rings, zig-zag rings, expansible box constructions, and the like. Each of these configurations may also be provided as either a self-expanding prothesis, typically being formed from an alloy displaying superelastic and/or shape memory properties, such as nickel titanium, or as a malleable prosthesis which is deployed by internal expansion of a balloon to radially expand the prosthesis structure.
Of particular interest to the present invention, are serpentine, zig-zag, and box stents which comprise a plurality of strut elements joined by malleable or plastically deformable hinge regions. Such prostheses are expanded by applying a radially outward internal force within a lumen of the prosthesis, typically by expanding a balloon catheter therein. As a result of such radially expansive forces, the hinge regions yield and the struts open away from each other to increase the diameter and peripheral dimension of the prostheses. While stents and other prostheses having such structures have been quite successful, they can suffer from non-uniform opening characteristics, as described in more detail below.
An exemplary prosthesis construction is shown in FIG. 1, where a serpentine stent 10 comprises struts 12 joined by hinge regions 14. The particular stent 12 illustrated includes a total of twelve struts 12 joined by twelve hinge regions 14, with six hinge regions being disposed at each end of the stent. Ideally, as the stent 12 is expanded by a balloon, each of the hinge regions 12 will open simultaneously at an equal rate so that the angles between adjacent struts 12 remain equal at all times during expansion. Unfortunately, even very small differences in the mechanical characteristics of the different hinge regions 12 can result in significantly different opening rates, as discussed below. Additionally, when a stent is deployed by a pre-folded balloon, certain internal surfaces of the stent may experience greater tangential forces than experienced by other surfaces, causing an uneven expansion.
FIGS. 2 and 3 illustrate stent 10 in a "rolled-out" view. FIG. 2 shows the stent in its non-expanded configuration prior to deployment. FIG. 3 shows the stent in a typical configuration after partial opening as a result of internal balloon expansion. The uneven pattern of FIG. 3 will result when hinge regions 14a are mechanically stronger than hinge regions 14b, resulting in hinge regions 14b opening more readily than hinge regions 14a. A similarly uneven expansion pattern may also result from non-uniform tangential forces between the balloon and the interior surfaces of the stent, or from a combination of these forces and differences in the strengths of the hinge regions. It will be appreciated that such non-uniformity results in a very poor distribution of support about the periphery of the body lumen being treated. As illustrated in FIG. 3, approximately one-half of the periphery of the body lumen would be supported by the four struts 12b, while the remaining one-half of the periphery would be supported by eight struts 12a. While the poor distribution of stent opening may be at least partially reduced as the stent 10 is further expanded, in many cases the stent 10 will only be partially opened after the deployment is complete. In such cases, the non-uniform strut distribution pattern will be a significant problem.
A separate problem in stent construction and deployment relates to the ability to detect the stent fluoroscopically during the deployment procedure. Stents composed of nickel titanium alloys and other radiopaque materials can be readily observed fluoroscopically if the cross-sections of their components are sufficiently large. If the stent is highly radiopaque, however, the stent itself, even in its expanded condition, can interfere with subsequent fluoroscopic examination of the treated area to confirm that the body lumen remains patent. In contrast, stainless steel and other common stent materials are generally radiolucent, i.e. they permit fluoroscopic examination therethrough. Such stents are advantageous since they do not prevent subsequent fluoroscopic examination of the treated region of the body lumen. They are, however, much more difficult to position accurately and usually require attachment of a separate radiopaque marker(s).
For these reasons, it would be desirable to provide improved stents and other lumenal protheses. In particular, it would be desirable to provide radially expansible stents and prostheses comprising strut and hinge regions, where the opening characteristics of the struts may be "programmed" to assure uniformity. Additionally, it would be desirable to provide improved stents and other prostheses having radiopacity characteristics which permit both tracking during deployment and subsequent visualization of the treated lumen after deployment. The present invention will provide at least some of the desired improvements.
2. Description of the Background Art
EP 662 307 describes an expansible stent having serpentine elements having regions with varying degrees of curvature to provide controlled expansion characteristics. EP 679 372 describes an expansible stent which is plated with a radiopaque material at each end to enhance fluoroscopic visibility.
Copending application Ser. No. 08/463,166, filed on Jun. 5, 1996, describes a radially expansible stent which could employ the weakened hinge regions of the present invention to control deployment. The full disclosure of this application is incorporated herein by reference.